The conventional wisdom holds that a diesel particulate filter must be periodically regenerated in order to reduce back pressure on the engine and/or to prevent a runaway exothermic soot oxidation reaction in a soot cake trapped in the filter. Reducing back pressure on the engine is generally associated with more efficient operation, and hence an incremental reduction in fuel consumption by the engine. A runaway exothermic oxidation reaction is generally undesirable since temperatures can become briefly so high that the filter substrate (e.g., zeolite) may become cracked or otherwise damaged to the point that the filter may be compromised. The threshold soot load density in the diesel particulate filter at which a runaway exothermic oxidation reaction might occur is reduced when the diesel particulate filter is coated with a diesel oxidation catalyst as in many conventional systems. The diesel oxidation catalyst serves to catalyze a reaction between nitrogen oxide in the exhaust with available oxygen to produce nitrogen dioxide. Active regeneration of a diesel particulate filter refers to a process by which the accumulated soot in the diesel particulate filter is oxidized by increasing the temperature at the filter in order to encourage soot oxidation. The active regeneration process is sometimes carried out with fuel injected into an aftertreatment system upstream from the diesel particulate filter, or by the use of electrical heaters or the like. By initiating the regeneration process at a relatively low soot load density, the oxidation reaction can be controlled, and a runaway exothermic reaction, and the damage risks associated with such a reaction, can be avoided. There is often a trade off between the additional fuel consumption necessary to perform active regeneration of the diesel particulate filter verses the additional fuel needed by the engine to overcome back pressure associated with a soot accumulation on the diesel particulate filter.
Apart from treating soot, most aftertreatment systems also attempt to reduce the presence of NOx at the tailpipe by catalyzing a NOx reduction reaction with an added reductant, such as urea. In many conventional systems, urea is injected into the aftertreatment system downstream from the diesel particulate filter. After mixing with the exhaust, a chemical reaction is encouraged with a NOx reduction catalyst to convert nitrogen dioxide and ammonia from the urea into nitrogen and water before exiting the tailpipe. In general, the amount of urea injected into the aftertreatment system must balance the amount of NOx present in the exhaust in order to avoid an inadequate reaction producing NOx at the tailpipe (NOx slip) or too much injection resulting in ammonia undesirably leaving the tailpipe (ammonia slip). In order to consume less urea, the conventional wisdom has generally been to adjust the engine calibration to produce less NOx while otherwise still meeting the demands on the engine. In general, NOx production increases with increased combustion temperature, as does engine efficiency. Therefore, adjusting an engine calibration to produce less NOx generally results in a reduction in engine efficiency, and hence an associated incremental increase in fuel consumption. Thus, the tradeoff with regard to NOx often relates to an incremental increase in fuel consumption in order to generate less NOx at the time of combustion along with a reduced demand for urea injection in order to arrive at a balanced reduction reaction.
The conventional wisdom has thus been a search for engine calibrations, diesel particulate filter regeneration frequency and urea injection quantities that result in an overall liquid consumption (fuel plus urea) that is acceptable while meeting emissions regulations. These strategies are typically carried out with an aftertreatment system that includes, in series, a fuel injector to facilitate regeneration, a diesel particulate filter coated with a diesel oxidation catalyst, a urea injection system, a mixer, and finally a NOx reduction catalyst.
Although the conventional aftertreatment system structure has seen success and become somewhat widespread in use, there have been efforts to locate the NOx reduction reaction at the diesel particulate filter by coating the same with a NOx reduction catalyst. For instance, published U.S. Patent Application 2010/0058746 teaches a diesel particulate filter coated with both a diesel oxidation catalyst and a NOx reduction catalyst. However, this reference teaches a necessity of frequent active regeneration of the diesel particulate filter.
The present disclosure is directed to an alternative aftertreatment strategy in conjunction with an engine system that can effectively compete with conventional aftertreatment system designs.